Flood in Alaska, August 18 to 20, 2022

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 AM AKDT Thu Aug 18 2022

.ANALYSIS AND UPPER LEVELS...

A upper level low continues to meander over the Y-K Delta, with a
trough axis extending eastward across Southcentral Alaska. The
entire trough, as well as embedded shortwaves, continues to
rotate northward around the east side of the upper level low. One
such shortwave brought a decent dose of rain to Southcentral last
afternoon into the evening. The Anchorage Bowl saw anywhere from
0.25 inches to as much as 1.00 inch of rain from the system. At
the surface, ridging extends along the Southcentral coast which is
enhancing gap winds through Turnagain Arm and the Copper River
Valley. Radar over Southwest shows showers continuing to taper
off while another shortwave moves northeastward towards the Prince
William Sound area. Due to this, the Middleton Island radar shows
rain filling in this morning.

Further west, a shortwave ridge is present over the central
Aleutians, with a large intensifying low moving eastward in behind
from the northwest Pacific into the southwest Bering Sea.
Lightning strikes are being detected around the center of
circulation. Areas of rain and gusty winds from small craft
advisory to potentially gale force are spreading eastward across
the western and central Bering Sea and Aleutians.

&&

.MODEL DISCUSSION...

Models remain in reasonably good agreement with the overall
synoptic pattern and general placement and intensity of features
in the short term. However, models diverge on solutions when it
comes to handling timing and placement of shortwaves. One aspect
that all models agree on is that Southcentral Alaska, especially
coastal locations, looks to be locked in to a wet pattern with
very few breaks in precipitation events. Overall forecast
confidence is high, with the best confidence in the synoptic and
lower confidence with the finer details of mesoscale features.

&&

.AVIATION...

PANC...VFR conditions are expected for much of the day before
another system moves into the region this evening. This will bring
another round of rain with lowering ceilings to MVFR and
potentially MVFR conditions for visibility, especially in heavier
rain. Also, gusty southeasterly winds will continue through the
morning hours today before diminishing by this afternoon.
Confidence is high for the Turnagain Arm winds through around 18/20z.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Today through Saturday night)...

The cool and rainy pattern will continue for the foreseeable
future. However, there will be breaks in the rain over the next
couple of days. A low pressure system over western Alaska will
push a shortwave east today which will become negatively tilted.
This will provide lift and with south to southwest flow from the
surface into the upper levels rain will redevelop from the western
Kenai Peninsula and towards Anchorage and the MATSU valleys later
this morning and into the afternoon. Another shortwave moving
northeast across the Gulf today will increase rain across coastal
areas including Prince William Sound. Up-inlet flow will continue
today with south winds expected across most valleys and gusty
southeasterly winds across west Anchorage, the Turnagain Arm and
the Copper River valley. Down-inlet flow will develop tonight with
winds subsiding across the valleys. As the shortwave swings east
tonight there may be brief breaks in the rain across Anchorage and
the MATSU valleys. But the rain will fill in pretty quickly on
Friday as a frontal system pushes north across Kodiak and the
western Gulf. Winds will also increase across the Kamishak Gap and
Barren Islands where winds near Gale force are possible on
Friday. As we head into Saturday the front will stall out near the
Kenai Peninsula as a ridge builds across southeast Alaska towards
the interior. This will keep the bulk of the rain from the MATSU
valleys and southward including Prince William Sound while the
Copper River Basin will be dry.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Weak upper level troughing exists over Southwest Alaska today.
Pockets of light rain may persist through the morning hours,
mainly over the Kuskokwim Valley, but will taper off into the
afternoon. Elsewhere will be relatively dry, with some breaks in
the cloud cover today and tonight. A front approaches the
Southwest Coast Friday morning, bringing with it another round of
rain showers to the region and increasing cloud cover. Moderate
to heavy rainfall is expected across Bristol Bay through Friday
afternoon and should lighten up by Friday night. Warm, south-
southeasterly flow will help gradually increase temperatures over
the next couple of days, reaching into the upper 50's on Friday
and low 60's on Saturday. Additionally, as the front pushes
onshore winds shift to easterly and become gusty, especially as
they filter through the mountain passes into Bristol Bay, on
Friday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Large low pressure system pushes southeast across the Bering Sea
over the next few days. Widespread gale force winds track along
the front as well as along the backside of the low through Friday
evening, with periods of storm force gusts. Along the front,
expect widespread heavy rain showers today, becoming lighter by
tonight. Heavy rain showers will continue tonight along the
Eastern Aleutians, southern Alaska Peninsula, and into Bristol
Bay. Additionally, isolated thunderstorms are possible across the
western Bering through this morning.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...

Gulf of Alaska: A front lifts northward from the western Gulf over
the weekend. With the exception of gap winds near the Barren
Islands and in Kamishak Bay, winds overall are expected to be
less than 35 kts and seas less than 20 ft. Aleutians/Bering: An
area of low pressure reaches south of the eastern Aleutians on
Saturday. Some northerly gap winds may persist across the central
and eastern Aleutians through at least Sunday, but are expected to
be less than 35 kts. Seas approach 25 ft on the Pacific side of
the western Aleutians on Saturday before subsiding for the rest of
the day.

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...

Looking ahead in the extended forecast, the latest operational
model runs are in a decent agreement, but begin to diverge in the
specific details towards the end of the period. Over the weekend,
a broad upper low crosses the Bering Sea and Aleutian Islands
before stalling out early next week. An associated surface low
positioned south of the Alaska Peninsula (AKPEN) will become
vertically stacked and have a slow southward progression. Ahead of
the surface low, southerly winds will draw ample moisture to
produce unsettled conditions for the AKPEN and Southwest Alaska
for most of the week. For Southcentral, a brief break of drier
conditions is possible for the inland areas between Sunday to
Monday, while coastal areas will continue to see rain along and
ahead of the low’s front. As the front lifts northward and pushes
inland, rain returns for the southern mainland as the atmospheric
river becomes established. However, timing of the precipitation
will be the main issue as models struggle to get handle on upper
level waves ejecting ahead of the aforementioned upper low. Out
west, after a quick moving storm system crosses the western
Aleutians on Sunday, a ridge of high pressure develops and expands
eastward with stratus and fog prevailing over the Bering Sea and
Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Special Weather Statement 101.
MARINE...Gale Warning 411 412 413 176 177 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DN
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...MF

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1154 AM AKDT THU AUG 18 2022

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1133 AM     HEAVY RAIN       8 ESE VALDEZ            61.08N 146.16W
08/18/2022  E0.25 INCH       CHUGACH            AK   COUNTY OFFICIAL

            RAIN FALL SINCE MIDNIGHT. PREVIOUS HEAVY
            RAIN. MILE 4.5 RICH HWY RIVER RUNNING HIGH.
            WIDESPREAD OFF-ROAD PONDING AND DITCHES
            RUNNING FULL.

&&